Heterogeneous-catalysed direct transformation of microalga biomass into Biodiesel-Grade FAMEs

•FAME obtained from direct heterogeneous-catalysed transformation of microalgae.•An analysis of the process variables was carried out to obtain a high FAME yield.•A purification step is necessary to meet biodiesel standards.•A two-step purification method that combined wet and dry treatments is studied.

The production of biodiesel using microbial lipids is attractive as oleaginous microorganisms can accumulate significant amounts of lipids (>20%) and they do not compete with food production. However, the poor economic viability of the production process involving cell harvest, microalga oil extraction, lipid transesterification and esterification reactions as well as biodiesel purification leads to a non-viable biodiesel production process. Thus, lipid transesterification and esterification optimization in addition to biodiesel purification, are important limiting factors for this process. The present research is focused on the optimization of the reaction conditions in order to obtain a high conversion to FAMEs in the direct biodiesel production from the microalgae N. gaditana using the CT-269 ion-exchange resin. The process was analysed by following the factorial design and response surface methodology concluding that optimum values of the variables are a temperature of 95 °C, a mass ratio of catalyst to microalga of 0.52/1, and a mass ratio of methanol to microalga of 33/1. Besides, the properties of the crude biodiesel were evaluated according to EN 14214 and ASTM D6751 standards, indicating that it is necessary an additional downstream purification step to remove all components that defile the final biodiesel. After an a two-step purification method that combined wet and dry treatments, ASTM D6751 biodiesel grade-FAMEs were obtained for the properties evaluated. The biodiesel also met all the properties determined according to EN 14214 except for the content of polyunsaturated FAMEs, the iodine value, oxidation stability and the cetane number.

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